What is the ruling on money that a father deposited for his son in the bank, which multiplied due to bank interest, with the son knowing the impermissibility of usurious interest, and is the son sinful if he only takes the principal?

The believer is commanded to seek out what is lawful. Investment certificates with a fixed, guaranteed return are considered forbidden usury (riba). You must dispose of the profits you acquired after learning of their unlawfulness by giving them in charity to the poor or for the benefit of Muslims. As for the profits gained before you knew of their unlawfulness, it is more cautious to dispose of them for charitable purposes, unless you are poor and in need. Some jurists have held that one who entered into a usurious contract unknowingly of its prohibition may benefit from what they earned.